A wallpaper should:

-be consistent in terms of style with the other desktop artwork.

-appear rather neutral in terms of taste. In other words a default wallpaper should be appropriate for both corporate and home use.

-use the current Oxygen color palette.

-not be distracting. This means that it shouldn't be too colorful and shades shouldn't change too rapidly. This is especially valid for the upper left area.

-be designed with the desktop icons and the kicker in mind. Ideally a default wallpaper should show just a single color on the upper 30%, the left 30% of the screen and the kicker area. The reason is that people place their icons in these places and having a single color without any other shapes and picture elements makes it easier to recognize them. Simplicity in these areas is very important.